1. Field of the Invention
The present invention relates to an electric power tool capable of applying the intermittent impact, percussion in the axial direction and the like to an anvil protruding to the front of a housing by selecting an operation mode.
2. Description of the Related Art
As an electric power tool, an impact tool described in Japanese laid-open patent application No. 2000-317854 is well known. In this application, rotation of an output shaft of a motor is transmitted to a driving shaft in a housing through a planetary gear reduction mechanism, and a hammer biased forward by a coil spring is externally provided with the driving shaft through a ball. Then, by engaging the hammer with an arm of an anvil (an output shaft) protruding to the front of the housing, rotation of the driving shaft is transmitted to the anvil through the hammer. With this structure, when a load on the anvil increases, the hammer moves backward by rolling of the ball to temporarily disengage from the arm of the anvil, and thereafter it moves forward by biasing of the coil spring to reengage with the arm. With this operation of the hammer, it is possible to apply the intermittent impact operation to the anvil (impact mode).
In addition, in this impact tool a drill mode in which impact operation by an impact mechanism is released to eliminate impact to the anvil can be selected. In a releasing means, a carrier of the last stage of the planetary gear reduction system is provided movably in the axial direction so as to be moved by an operating member from outside. The carrier is connected with a connecting member through a switching pin penetrating the center of axle of the driving shaft. The connecting member serves as a switching member which can engage with both the driving shaft and the anvil. With this configuration, the carrier is moved by the operating member to a sliding position to engage with both the driving shaft and the anvil, thereby the driving shaft and the anvil are incorporated.
On the other hand, a percussion drill having a percussion mechanism described in Japanese laid-open utility model publication No. S51-14389 is well known. In this percussion drill, a spindle (an output shaft) rotating driven by a motor is provided so as to be slightly moved back and forth in the axial direction, and the spindle is biased to a forward position by a biasing means such as a coil spring externally provided with the spindle. The spindle is provided with a first clutch which rotates integrally therewith, while a housing is provided with a second clutch into which the spindle is inserted with play for facing the first clutch. When the spindle is moved backward by pressing a bit mounted thereon, the first clutch engages with the second clutch, whereby percussion is applied to the spindle in the axial direction.
Upon mounting of the bit to the spindle, a chuck provided with the spindle is used as disclosed in the Japanese laid-open utility model publication No. S51-14389. Besides, such a structure is often used that a chuck sleeve externally mounted to the end of the spindle is provided so as to be movable back and forth with a predetermined stroke in the axial direction, and the chuck sleeve is biased to either forward or backward direction by a biasing means such as a coil spring. At the biased sliding position, a pressing member internally provided to the spindle so as to be movable in the radial direction, a ball for example, is pressed to the side of the center of axle of the spindle, thereby fixing the bit inserted into an attaching hole which is provided with the spindle. When the chuck sleeve is slid in the opposite direction against the biasing force, the pressing member pressed by the chuck sleeve is released and the bit can be mounted or detached.
In addition to the impact mode and the drill mode, a clutch mode (driver mode) can be applied to an impact tool, in which rotation transmission is stopped at a predetermined torque to an anvil. For example, this structure can be obtained by causing one of internal gears to be rotatable in the planetary gear reduction mechanism between the motor and the output shaft, and providing a pressing means for pressing the internal gear by a coil spring through a ball and a washer etc. which engage with the end of the internal gear. That is, when a load to the anvil exceeds to a biasing force of the coil spring, the internal gear is caused to idle to stop rotation transmission to the anvil.
On the other hand, besides the impact mode and the drill mode, a percussion drill mode applying percussion in the axial direction to the anvil can be applied. For example, this structure can be obtained by causing an anvil to be slightly movable back and forth and biased to a forward position in a normal state. When the anvil is at a backward position, cams provided with both the anvil and the housing engage with each other, thereby percussion is applied to the anvil.
Accordingly, when the clutch mode or percussion drill mode is applied, a switching means for switching between the drill mode and the above modes is further required. For example, in the clutch mode, a structure is applied that an operation means such as a change ring is rotated to slide the switching means which can engage with the internal gear between the engaging position and the disengaging position, so that regulation of the internal gear rotation and its release can be selected. On the other hand, in the percussion drill mode, a structure is applied that when one cam is fixed to the anvil and the other cam is made to be rotatable in the housing, a switching means which can engage with the rotatable cam is slid between the engaging position and the disengaging position by an operating means, so that percussion and its release can be selectively applied to the anvil.
When the selectable modes are thus increased, an impact switching member for switching between an impact mode and a drill mode, a clutch switching member for switching between the drill mode and the clutch mode, and a percussion switching member for switching between the drill mode and a percussion drill mode have to be separately manufactured, so that operability is deteriorated and malfunction might occur.
On the other hand, in the percussion drill mode a biasing means for biasing the spindle to a forward position and another biasing means for the chuck sleeve are separately provided. As a result, the number of parts increases and thus structure is complicated, which makes assembly troublesome and the cost high.
In view of the above, an object of the present invention is to provide an electric power tool which prevents malfunction with excellent operability even when one operation mode among various modes are selectable and in which the output shaft and the chuck sleeve are rationally biased to simplify the structure and achieve the lower cost.